Boot the one that works, open a CMD.EXE window.
Here's your command sequence :
C:
attrib -r -h -s boot.ini
edit boot.ini
attrib +r +h +s boot.ini
When you edit the boot.ini file itself you can see which partition and stuff is selected; the entries in this file are the same order as the boot menu, so just remove the one you DIDN'T use to boot the system :)